1. Field of the Invention
This invention generally relates to shaft seals, and is specifically concerned with a hydrostatic, radially tapered film-riding seal which is useful in sealing the shaft of the coolant pump used in a nuclear power plant.
2. Description of the Prior Art
Hydrostatic, radially tapered "film-riding" seals are known in the prior art. In the coolant pumps utilized in the primary heating systems of nuclear power plants, such seals allow the pump shafts to transmit torque through the pressurized interior of the pump housing with a minimal amount of friction and water "leak-off". In the past, such seals have utilized a sealing ring and a runner formed from alumina which include sealing surfaces tapered at specific angles in order to form a hydrostatic film between the sealing surface of the ring and sealing surface of the runner. This hydrostatic film allows the ring and the runner to operate in a non-rubbing mode. Unfortunately, such prior art shaft seals are not without shortcomings. However, in order to fully appreciate these shortcomings, at least a general understanding of the structure and operation of the cooling pumps which form the environment of such seals is necessary.
Nuclear power plants which incorporate the general design pioneered by Westinghouse Electric Corporation have separate primary and secondary hydraulic systems. Generally speaking, the primary system includes a coolant pump for circulating water from the hot, radioactive reactor core through the interiors of a plurality of U-shaped tubes mounted in the tubesheet of a nuclear steam generator. Water from the secondary system flows over the outside surfaces of these U-shaped tubes and absorbs the heat radiated and conducted through the walls of the heat exchange tubes. This water from the secondary system ultimately forms steam which is used to drive the turbines of the nuclear power plant, and generate electricity. Because the water in the primary system is hydraulically insulated from the water in the secondary system by the heat exchange tubes, the Westinghouse system allows non-radioactive water to be used to generate the steam which ultimately spins the turbines of the electric generators.
In order to generate an optimum amount of electric power from the hot, radioactive reactor cores, it is necessary to use a coolant pump to rapidly circulate water within the primary system from the reactor core to the interiors of the U-shaped tubes in the nuclear steam generator, and back again. Consequently, the water within the housing of the coolant pump is typically about 550.degree. F., and pressurized to a level of approximately 2,250 psi. Because the pump shaft of the coolant pump is rotatably mounted across the pressure boundary between the interior of the pump housing and the outside atmosphere, the pressure differential tends to bias the pump shaft out of the pump housing in much the same way a hot gas pushes a piston out of a cylinder. Prior art shaft seals were developed in order to allow the pump shaft to transmit torque through the pressurized pump casing with a minimum of friction, and a minimum of water leakage through the casing. Such shaft seals generally comprised a sealing ring mounted onto the pump casing, and a runner mounted on top of a flange of the pump shaft. The pressure within the pump housing placed a compressive load between the sealing ring and the runner; however, this compressive load was counteracted by a flowing film of water between the sealing surfaces of the ring and the runner. Such a film was generated by providing a radial taper on the sealing surface of the sealing ring, so that it was very shallow frustro-conical, rather than flatly horizontal. Pressurized water surrounding the annular space between the two sealing surfaces would create a film of flowing water approximately one-half of one mil thick, which would allow the sealing ring and the runner to "ride" upon one another without a frictional or rubbing contact.
One of the potential problems associated with the seals used in such coolant pumps is erosion. The water in the primary system contains abrasive particles (as a result of the corrosion which takes place within the system) which can scratch the sealing surfaces if these particles are allowed to flow between these surfaces. In order to prevent abrasive, particulate matter in the primary system from flowing through the narrow space between the sealing surfaces of the ring and the runner, injection pumps have been provided on such coolant pumps which pump cooler, filtered water into the region around the seal at a slightly higher pressure than the primary water present within the balance of the pump housing. The provision of such injection pumps also allows the resulting "leak-off" which occurs as a result of the creation of the fluid film to be non-radioactive, which in turn minimizes potential radiation contamination within the plant.
While the aforementioned prior art shaft seals operate satisfactorily under normal conditions, severe damage can result to the seals under a variety of emergency conditions. If, for example, the pressure within the pump housing should fall below about 2,000 psi, the seal may cease to operate in a film-riding mode and start to operate in a frictional or "rubbing" mode. In alumina seals, if this should occur for more than 30 seconds, the seal will be irreparably damaged. Additionally, the flow speed of the film of water between the sealing surfaces varies substantially with fluctuations in the pressure of the water within the pump housing. Thus, in an over-pressure condition, the film speed can become high enough to generate a turbulent flow between the sealing surfaces. Since a turbulent flow generates a film which is far less stiff than a laminar fluid flow, the sealing surfaces may start to operate in the previously mentioned rubbing mode, which in turn will destroy the seal. Even if the seal is not destroyed, such a high film speed may cause the amount of water "leak-off" to become unacceptably high. On the other hand, should the pressure drop below a certain point, the seal will self-destruct as a result of the loss of a film of sufficient "stiffness" to separate the sealing surfaces. A further shortcoming of seals having sealing rings and runners formed from alumina is their tendency to erode in the event the injection pumps fail. Under such circumstances, abrasive, particulate matter from the primary water system passes through the narrowly adjacent sealing surfaces and scratches them. If the operation of the injection pumps is not quickly restored, the sealing surfaces can become irreparably damaged. In all cases, if the seal is destroyed, an expensive shut-down of the plant is necessary before the seal can be replaced.
Clearly, there is a need for an improved shaft seal which is capable of operating in a rubbing mode should the pressure in the coolant pumps fail. Ideally, such a shaft seal should have a fairly constant film flow rate, and low "leak-off" despite wide variations in the pressure of the water within the pump housing. Finally, such a seal should have good anti-erosive characteristics should the failure of the injection pumps allow abrasive, particulate matter from the primary system to flow between the sealing surfaces of the ring and runner.